The isotopomer ratios of N2O in the Shaying River, the upper Huai River network, Eastern China: The significances of mechanisms and productions of N2O in the heavy ammonia polluted rivers

Dissimilatory nitrate reduction pathways drive high nitrous oxide emissions and nitrogen retention under the flash drought in the largest freshwater lake in China

Flash drought (FD) events induced by climate change may disrupt the normal hydrological regimes of floodplain lakes and affect the plant-microbe mediated dissimilatory nitrate reduction (DNR), i.e., denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA), thus having important consequences for nitrous oxide (N2O) emissions and nitrogen (N) retention. However, the responses of the DNR pathways in the floodplain lake to the record-breaking FD in 2022 in Yangtze River of China, as well as the underlying microbial mechanisms and feedbacks to climate change remain poorly understood. Here, we collected exposed sediments and Carex cinerascens-associated soils in the littoral wetlands of Poyang Lake during 2022 FD and the dry seasons prior to and after this event. The potential DNR rates and the synergistic metabolism of microbial guilds involved in DNR were investigated using 15N isotope pairing technique, high-throughput and metagenomic sequencing. We found that the in situ N2O fluxes in the littoral wetlands were highest during the flash drought, especially in the exposed sediments. The potential DNRA rates were highest under flash drought conditions, and DNRA dominated the DNR for both exposed sediments (80.4 %) and Carex cinerascens-associated soils (57.5 %). Nutrients (i.e., N and P) and DNRA bacterial communities played a key role in producing the extremely high N2O fluxes from exposed sediments, which could be explained by the synergistic metabolism of DNRA bacteria and denitrifiers through the exchange of the key intermediates in DNR. Therefore, the climate change-induced flash drought promoted greater nitrous oxide emissions and N retention in the littoral wetlands of Poyang Lake, producing a greater flux of greenhouse gas emissions and elevating the risk of lake eutrophication. Hence, flash droughts reinforce a positive feedback between climate change and nitrous oxide emission from these aquatic ecosystems.

New insights into nitrate sources and transformations in riparian groundwater of a sluice-controlled river: An integrated approach using major ions, stable isotopes and microbial gene methods

Nitrate pollution in water environment is a serious problem worldwide. Identifying nitrate sources and transformations in the riparian aquifer is critical for effectively controlling and mitigating nitrate contamination, especially in sluice-controlled rivers. This study employs an integrated approach combining hydrochemical analysis, isotopes (δ18O-H2O, δ2H-H2O, δ15N-NO3- and δ18O-NO3-), quantification of nitrogen (N) functional genes and a Bayesian mixing model (MixSIAR) to comprehensively investigates nitrate sources and transformation processes in the riparian groundwater of a sluice-controlled Shaying River, China. Results revealed severe nitrate contamination in both the river (mean: 2.33-5.25 mg/L) and the riparian groundwater (mean: 0.42-24.46 mg/L). Manure and sewage were the primary sources (66.20-91.20 %) of nitrate contamination in both river and riparian groundwater. Key processes influencing nitrate dynamics in riparian groundwater included mixing with river water, external N supply, and transformation processes such as nitrification, vegetation uptake and anammox. We found that when sluices are closed, the nitrate concentration in riparian groundwater decreases. In contrast, during the flood season with sluices open, the nitrate concentration in the river water increases. This study also developed the first conceptual model illustrating the impact of sluice regulation on riparian nitrate dynamics, highlighting the complex interplay between sluice operations, hydrological conditions, and biogeochemical processes that govern nitrate behavior. These findings provide valuable insights into nitrate dynamics in riparian aquifers of sluice-controlled rivers, offering a robust scientific foundation for targeted nutrient management strategies in the Shaying River Basin and similar regulated environments globally.

Labile organic matter favors a low N2O yield during nitrogen removal in estuarine sediments

Estuary harbors the active sediment denitrification and nitrous oxide (N2O) emission, while the knowledge of environmental controls on the denitrification-derived N2O yield remains underexplored. Here, we quantitatively assess the potential and in situ rates of N2O production during sediment denitrification in the Pearl River Estuary (PRE), China. Organic matter determines the product stoichiometry and capacity of nitrogen removal. In particular, labile organic matter (LOM) reduces N2O yield via enhancing the complete coupled nitrification-denitrification. Our results reveal that the chain processes, primary production-LOM settling-sedimentary respiration-coupled nitrification-denitrification, control the sediment denitrification and N2O production, linking the carbon and nitrogen biogeochemical cycles in the atmosphere-water column-sediment continuum. The PRE sediments serve as nitrogen removal hotspots but with low efficiency (~25 % of riverine input) and strong N2O release (~66 % of daily sea-air N2O efflux). These findings contribute to policy makers to develop knowledge-based management actions for achieving sustainable coastal environments and mitigating N2O emission.

Long-neglected contribution of nitrification to N2O emissions in the Yellow River

Rivers play a significant role in the global nitrous oxide (N2O) budget. However, the microbial sources and sinks of N2O in river systems are not well understood or quantified, resulting in the prolonged neglect of nitrification. This study investigated the isotopic signatures of N2O, thereby quantifying the microbial source of N2O production and the degree of N2O reduction in the Yellow River. Although denitrification has long been considered to be the dominant pathway of N2O production in rivers, our findings indicated that denitrification only accounted for 18.3% (8.2%-43.0%) of the total contribution to N2O production in the Yellow River, with 50.2%-80.2% being concurrently reduced. The denitrification contribution to N2O production (R2 = 0.44, p < 0.01) and N2O reduction degree (R2 = 0.70, p < 0.01) were positively related to the dissolved organic carbon (DOC) content. Similar to urban rivers and eutrophic lakes, denitrification was the primary process responsible for N2O production (43.0%) in certain reaches with high organic content (DOC = 5.29 mg/L). Nevertheless, the denitrification activity was generally constrained by the availability of electron donors (average DOC = 2.51 mg/L) throughout the Yellow River basin. Consequently, nitrification emerged as the primary contributor in the well-oxygenated Yellow River. Additionally, our findings further distinguished the respective contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N2O emissions. Although AOB dominated the N2O production in the Yellow River, the AOA specie abundance (AOA/(AOA + AOB)) contributed up to 32.6%, which resulted in 25.6% of the total nitrifier-produced N2O, suggesting a significant occurrence of AOA in the oligotrophic Yellow River. Overall, this study provided a non-invasive approach for quantifying the microbial sources and sinks to N2O emissions, and demonstrated the substantial role of nitrification in the large oligotrophic rivers.

Denitrification regulates spatiotemporal pattern of N2O emission in an interconnected urban river-lake network

Urban rivers are hotspots of N2O production and emission. Interconnected river-lake networks are constructed to improve the water quality and hydrodynamic conditions of urban rivers in many cities of China. However, the impact of the river-lake connectivity project on N2O production and emission remains unclear. This study investigated dissolved N2O and emission of the river-lake network in Wuhan City, China from March 2021 to December 2021. The results showed that river-lake connection greatly decreased riverine Nitrogen (N) concentration and increased dissolved oxygen (DO) concentration compare to traditional urban rivers. N2O emissions from the urban river interconnected with lakes (LUR: 67.3 ± 92.6 μmol/m2/d) were much lower than those from the traditional urban rivers (UR: 467.3 ± 1075.7 μmol/m2/d) and agricultural rivers (AR: 20.4 ± 15.3μmol/m2/d). Regression tree analysis suggested that the N2O concentrations were extremely high when hypoxia exists (DO < 1.6 mg/L), and TDN was the primary factor regulating N2O concentrations when hypoxia does not occur. Thus, we ascribe the low N2O emission in the LUR and AR to the lower N contents and higher DO concentrations. The microbial process of N2O production and consumption were quantitatively estimated by isotopic models. The mean proportion of denitrification derived N2O (fbD) was 63.5 %, 55.6 %, 42.3 % and 42.7 % in the UR, LUR, lakes and AR, suggested denitrification dominated N2O production in the urban rivers, but nitrification dominated N2O production in the lakes and AR. The positive correlation between logN2O and fbD suggested that denitrification is the key process to regulate the N2O production and emission. The abundance of denitrification genes (nirS and nirK) was much higher than that of nitrification genes (amoA and amoB), also evidenced that denitrification was the main N2O source. Therefore, river-lake interconnected projects changed the nutrients level and hypoxic condition, leading to the inhibition of denitrification and nitrification, and ultimately resulting in a decrease of N2O production and emission. These results advance the knowledge on the microbial processes that regulate N2O emissions in inland waters and illustrate the integrated management of water quality and N2O emission.

Identifying surface water and groundwater interactions using multiple experimental methods in the riparian zone of the polluted and disturbed Shaying River, China

Identifying groundwater (GW)-surface water (SW) interactions in riparian zones is important for assessing the transport pathways of pollutants and all potential biochemical processes, particularly in rivers with artificially controlled water levels. In this study, we constructed two monitoring transects along the nitrogen-polluted Shaying River, China. The GW-SW interactions were qualitatively and quantitatively characterized through an intensive 2-y monitoring program. The monitoring indices included water level, hydrochemical parameters, isotopes (δ¹⁸O, δD, and ²²²Rn) and microbial community structures. The results showed that the sluice altered the GW-SW interactions in the riparian zone. A decrease in river level occurs during the flood season owing to sluice regulation, resulting in discharge of riparian GW into the river. The water level, hydrochemistry, isotopes, and microbial community structures in near-river wells were similar to those in the river, indicating mixing of the river water with the riparian GW. As the distance from the river increased, the percentage of river water in the riparian GW decreased, whereas the GW residence time increased. We found that nitrogen may be easily transported through the GW-SW interactions, acting as a sluice regulator. Nitrogen stored in river water may be removed or diluted by mixing GW and rainwater during the flood season. As the residence time of the infiltrated river in the riparian aquifer increased, nitrate removal increased. Identifying the GW-SW interactions is crucial for water resource regulation and for further tracing the transport of contaminants such as nitrogen in the historically polluted Shaying River.

Dissolved N2O concentrations in oil palm plantation drainage in a peat swamp of Malaysia

Oil palm plantations in Southeast Asia are the largest supplier of palm oil products and have been rapidly expanding in the last three decades even in peat-swamp areas. Oil palm plantations on peat ecosystems have a unique water management system that lowers the water table and, thus, may yield indirect N₂O emissions from the peat drainage system. We conducted two seasons of spatial monitoring for the dissolved N₂O concentrations in the drainage and adjacent rivers of palm oil plantations on peat swamps in Sarawak, Malaysia, to evaluate the magnitude of indirect N₂O emissions from this ecosystem. In both the dry and wet seasons, the mean and median dissolved N₂O concentrations exhibited over-saturation in the drainage water, i.e., the oil palm plantation drainage may be a source of N₂O to the atmosphere. In the wet season, the spatial distribution of dissolved N₂O showed bimodal peaks in both the unsaturated and over-saturated concentrations. The bulk δ¹⁵N of dissolved N₂O was higher than the source of inorganic N in the oil palm plantation (i.e., N fertilizer and soil organic nitrogen) during both seasons. An isotopocule analysis of the dissolved N₂O suggested that denitrification was a major source of N₂O, followed by N₂O reduction processes that occurred in the drainage water. The δ¹⁵N and site preference mapping analysis in dissolved N₂O revealed that a significant proportion of the N₂O produced in peat and drainage is reduced to N₂ before being released into the atmosphere.

Quantitative identification of nitrate and sulfate sources of a multiple land-use area impacted by mine drainage

The rapid increase in urbanization and intensive coal mining activities have accelerated the deterioration of surface water quality. Environmental problems caused by the accumulation of nitrate and sulfate from natural, urban, and agricultural sources have attracted extensive attention. Information on nitrate and sulfate sources and their transformations is crucial for understanding the nitrogen and sulfur cycles in surface water. In this study, we monitored nitrate and sulfate in three representative rivers in mining cities in northern China. The main pollution sources and biogeochemical processes were identified by using stable isotopes (δD, δ¹⁸O_H2O, δ¹⁵N, δ¹⁸O_NO3, δ³⁴S and δ¹⁸O_SO4) and hydrochemistry. The contribution of natural and anthropogenic sources was quantitatively estimated based on a Bayesian mixed model. The results indicated a large variation in sulfate and nitrate sources between the different rivers. Nitrate in the Tuohe River mainly derived from manure/sewage (57.9%) and soil N (26.9%), while sulfate mainly derived from manure/sewage (41.7%) and evaporite dissolution (26.8%). For the Suihe River, nitrate was primarily sourced from chemical fertilizer (37.9%) and soil nitrogen (34.8%), while sulfate was mainly sourced from manure/sewage (33.1%) and chemical fertilizer (21.4%). For the Huihe River, nitrate mainly derived from mine drainage (56.6%) and manure/sewage (30.6%), while sulfate predominantly originated from mine drainage (58.3%) and evaporite dissolution (12.9%). Microbial nitrification was the major pathway for the migration and transformation of nitrate in the surface water. However, denitrification and bacterial sulfate reduction (BSR) did not play a significant role as aerobic conditions prevailed. In this study, we elucidated the sources and transformation mechanisms of nitrate and sulfate. Additionally, we provided a reference for formulating a comprehensive strategy for effective management and remediation of surface water contaminated with nitrate and sulfate in mining cities.

Nitrate sources and transformations in surface water of a mining area due to intensive mining activities: Emphasis on effects on distinct subsidence waters

The increase in NO₃^- content in surface water caused by intensive mining activities in Huainan City, China, has attracted considerable attention owing to the deterioration of water quality and the degradation of ecosystems in recent years. The Huainan mining area, which is highly disturbed by anthropogenic activities, was selected as a typical observation area, and the surface water was classified as open subsidence water (OSW), closed subsidence water (CSW), and river water (RW). Moreover, the hydrochemical parameters and the δ¹⁵N and δ¹⁸O values of nitrate were employed to quantitatively trace the sources and biochemical transformation of NO₃^-, and the contribution ratios of different NO₃^- sources were estimated using the stable isotope analysis in R based on the Bayesian model. There was evident nitrification in the study area, but no significant denitrification has occurred. A substantial portion of δ¹⁵N-NO₃^- demonstrated complex sources of NO₃^-. Compared with those of CSW, the NO₃^- compositions of the OSW approached to those of the RW due to river recharge and discharge, and were greatly affected by anthropogenic activities. The proportional contribution of manure and sewage in the OSW was found to be the highest with a mean value of 39.5 % ± 12.3 %, which was followed by that of mine drainage (mean: 22.1 % ± 13.1 %), chemical fertilizer (mean: 17.5 % ± 10.6 %), and soil organic nitrogen (mean: 17.5 % ± 11.6 %). In the RW, the highest mean contribution of manure sewage was 35.2 % ± 9.7 %, which was followed by that of chemical fertilizer (mean: 29.3 % ± 7.2 %), mine drainage (mean: 23.4 % ± 13.0 %), and soil organic nitrogen (mean: 10.9 % ± 8.3 %). In contrast, the contribution of chemical fertilizer to the CSW was the highest with a mean value of 33.9 % ± 13.6 %, which was followed by that of soil organic nitrogen (mean: 26.5 % ± 13.8 %), mine drainage (mean: 18.1 % ± 11.6 %). Therefore, NO₃^- in the surface water of the mining area primarily originates from chemical fertilizers and manure sewage. In addition, the contribution of mine drainage to nitrate in the study area indicates the potential impact of mining activities on surface water. These findings highlight the value of classifying different types of surface water in tracing NO₃^- contamination sources, and provide relevant theoretical basis for tracing nitrate sources in other areas.

Applying constructed wetland-microbial electrochemical system to enhance NH4+ removal at low temperature

NH₄⁺ removal at low temperature (<10 °C) has baffled researchers and engineers for decades. Bioelectrochemical process has been increasingly valued as a promising approach to enhance NH₄⁺ removal by both electrochemical and stimulated microbial processes. The feasibility and the mechanism of enhanced NH₄⁺ removal were investigated in Constructed Wetland-Microbial Electrochemical System (CW-MES) with different electrode spacings including Constructed Wetland-Microbial Fuel Cell (CW-MFC) and Constructed Wetland-Microbial Electrolysis Cell (CW-MEC) at low temperature. Solar cell panel was firstly implemented in CW-MEC to enhance NH₄⁺ removal. The low-temperature operation lasted for about four months, CW-MEC successfully enhanced NH₄⁺ removal while CW-MFC did not exhibit positive effect. The NH₄⁺-N removal efficiency of CW-MEC achieved 88.2 ± 7.0%, which was 11.7 ± 6.5% higher than conventional constructed wetland (CCW). The maximum NH₄⁺-N removal efficiency of CW-MEC achieved 100%. The average NH₄⁺-N mass removal rate was 436.02 mg m^-2 d^-1. It was found that NH₄⁺ was mainly removed by the nitrification-autotrophic denitrification process in CW-MES while it was mainly converted to NO₃^- in CCW. Ammoxidation and denitrification were both enhanced by electricity while NH₄⁺ was used as the main substrate for electricity generation. AOA (Candidatus Nitrosocosmicus) and NOB (Nitrospira) were the main contributors to nitrification. This study provided a cost-effective and sustainable method for electrochemically enhanced microbial NH₄⁺ removal at low-temperature and revealed the relevant mechanism.